This invention relates to a wet process of preparing fine, plate-shaped particles of a ferrite having the magnetoplumbite structure suitable for use in magnetic recording media.
In the field of magnetic recording there is a growing demand for enhancing the recording density. To meet such a demand, recently it has been proposed to employ a perpendicularly magnetizing recording method which can enhance the recording density by at least several times compared with the conventional recording methods in which the magnetic material in a magnetic tape or disc is magnetized in the direction parallel to the surface of the tape or disc, and the new recording method is under development into practical applications.
In the magnetic recording media for use in the perpendicularly magnetizing recording method, it is necessary that an easily magnetizing axial direction of the magnetic material is perpendicular to the surface of the magnetic tape or disc. In the magnetic tapes and discs under development for use in the new recording method, the methods of forming a magnetic coating film are broadly classified into the deposition type in which a suitable magnetic alloy such as Co-Cr is subjected to sputtering or vacuum evaporation and the traditional coating type using a slurry or paint containing a magnetic powder. At present the coating method using a magnetic paint is expected to be superior in respects of mass productivity, stability and economy. As the magnetic powder for this purpose, a particular interest is attached to Ba-ferrite, BaO.n(Fe.sub.2 O.sub.3) where n is usually 5-6, which is typical of ferrites having the magnetoplumbite structure. Ba-ferrite powders for use in the perpendicularly magnetizing recording method are required to be high in dispersibility and very small in particle size on condition that the powders do not become superparamagnetic, to have such a particle shape as will allow easy orientation and to be fairly low in the coercive force.
British patent application publication No. 2,108,100A shows a wet process of preparing generally hexagonal plate-shaped and fine particles (e.g. 1.5-2 .mu.m) of Ba-, Sr- and/or Pb-ferrites having the magnetoplumbite structure by hydrothermal treatment of an iron oxide powder and a compound or compounds of barium, strontium and/or lead in an aqueous alkali solution in the presence of an oxidizer. The iron oxide is either .gamma.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4, or (FeO).sub.x.Fe.sub.2 O.sub.3 (0&lt;x&lt;1), which can be oxidized to .gamma.-Fe.sub.2 O.sub.3. This British specification does not mention about the magnetic properties of the obtained ferrite powders.
In general ferrites having the magnetoplumbite structure as represented by Ba-ferrite are relatively high in the coercive force, and there is a tendency that the coercive force becomes higher as the particle size of the ferrite becomes smaller. For example, we have found the intrinsic coercivity iHc of Ba-ferrite powders about 0.3 .mu.m in particle size becomes as high as 3000-5000 oersteds. For use in the perpendicular magnetic recording method, ferrite powders of lower coercivity values are desirable. A suitable level of the coercive force of a ferrite powder for this use will differ depending on the material of the recording head (which may be a sintered ferrite, alloy or amorphous), but in general the range of 300 to 2000 oersteds is said to be suitable.